The myometrium contracts spontaneously throughout gestation. This activity has been confirmed experimentally in several species, including humans, by measuring increases in intra-amniotic pressure (IAP) or changes in myometrial electromyographic activity (EMG). Continuous IAP and EMG recordings in chronically catheterized sheep show the presence throughout most of gestation of "contractures," EMG events of long duration (5-7 min.) that produce a small increase in IAP. The duration of these contractures decreases, while the frequency increases, as labor approaches; Nathanielsz et al., Am. J. Obstet. Gynecol., 138: 653 (1980); Krishnamurti et al., J. Reprod. Fert., 64: 59 (1982).
In primates, short-term recordings of IAP in pregnant rhesus monkeys show the presence of episodes of increased IAP, and the frequency of these episodes seems to be higher in late gestation; (Corner et al., Am. J. Obstet. Gynecol., 85: 179 (1963)). Continuous recording of IAP in chronically catheterized rhesus monkeys maintained in restraint chairs indicates that there is a circadian pattern of change of both the frequency and the rise of IAP generated in late gestation (Harbert, Am. J. Obstet. Gynecol., 129: 401 (1977)). Discontinuous myometrial EMG recordings in cynamologous monkeys chronically implanted with electrodes also suggest the presence of circadian variation of the frequency of EMG signals, and a change in the duration of the signal throughout the day; (Germain et al., Am. J. Obstet. Gynecolo, 142: 513 (1982)). EMG recordings in pregnant humans have been limited to labor, or during induction of labor (Wolfs et al., Acta. Obstet. Gynecol. Scand. (Suppl)., 9, (1979)).
It has recently been discovered through the study of electromyographic activity (EMG) in pregnant rhesus monkeys that at least two types myometrial activity occur during pregnancy. One of these, named Type I and associated with labor and delivery, is characterized by frequent bursts (e.g., 10 to 45 per hour) of electrical activity that last from 0.5 to 1.2 minutes. The other type of EMG activity observed, named Type II, and associated with spontaneous myometrial contractions during the normal development non-labor and delivery stages of pregnancy, consists of fragmented series of discharges that last 2 to 15 minutes and occur up to six times an hour. The frequency of Type I EMG events when present generally show a circadian pattern of low amplitude with a maximum at night. This daily pattern is amplified during the 8-10 days preceding delivery.
Direct observation and manual analysis of polygraph tracings showed that EMG and IAP activity were always closely coordinated. There were no consistent differences between EMG recordings made at placental and extraplacental sites, nor between fundus and body of the uterus. The frequency of trains of potentials appeared to be proportional to IAP increase achieved, although this has not yet been subjected to rigorous quantitation. Combining EMG and IAP recordings show that activity formed distinct bursts.
Two types of episodes were distinguished: